1. Field of the Invention
The present invention relates to a power supply system of a display apparatus and a control method thereof, and more particularly, to a power supply system of a display apparatus and a control method thereof which reduces a cost of manufacturing by simplifying a circuit and consumes less power.
2. Description of the Related Art
In display apparatuses, power supply systems supply necessary power to each component of the display apparatuses. SMPS (Switched Mode Power Supply) related technology is rapidly developing with the SMPS becoming lighter, more compact, and more efficient compared to linear power supplies.
As shown in FIG. 4, a conventional SMPS comprises a fullwave rectifier circuit 100 to convert an external AC (Alternating Current) voltage into a DC (Direct Current) voltage; a smoothing circuit 200 to smooth an output voltage of the fullwave rectifier circuit 100; a PWM (Pulse Width Modulation) circuit 400; a switching transformer 500; an output circuit 600 to rectify each voltage induced by the secondary windings Ls1 and Ls2 of the switching transformer 500 and to output the rectified voltages; a sync signal supplier 800 to supply an AFC (Automatic Frequency Control) signal to the PWM circuit 400; and a feedback circuit to maintain the output voltage of the output circuit 600 in a regular voltage level.
In the above configuration, the operation of a conventional SMPS can be described as follows. Initially, an AC voltage (100V/220V) is rectified in the fullwave rectifier circuit 100, which is then smoothed through the smoothing circuit 200. Thus, the AC voltage is converted into a DC voltage. Then, the DC voltage is applied to a primary winding Lp of the switching transformer 500. At the same time, the output voltage of the smoothing circuit 200 is supplied to the PWM circuit 400.
The PWM circuit 400 comprises a PWM IC (Integrated Circuit) 410 to generate a PWM signal, and a transistor and a diode 420, where the transistor and the diode 420 are turned on/off according to the PWM signal from the PWM IC 410.
As the transistor is repeatedly turned on/off according to the PWM signal of the PWM IC 410, inductive electromotive force is generated and transferred from the primary winding Lp to the secondary windings Ls1 and Ls2 of the switching transformer 500.
An AC voltage is output from the secondary windings Ls1 and Ls2 of the switching transformer 500 and converted into DC voltages Vcc1 and Vcc2, respectively, through the output circuit 600. The output voltages (Vcc1 and Vcc2) of the output circuit 600 are supplied to each of the circuits in the display apparatus or dropped to voltage levels that correspond to various components such as a microcomputer 700, or a video amplifier.
One deficiency of conventional SMPSs is that elements of the circuit can be damaged and the life expectancy of the switching power circuit can be shortened if the output voltages Vcc1 and Vcc2 of the output circuit 600 exceed the average voltage level. Accordingly, a feedback circuit is generally utilized in the switching power circuit to maintain the output voltages Vcc1 and Vcc2 of the output circuit 600 at a reference voltage level.
The feedback circuit senses the output voltages Vcc1 and Vcc2 of the output circuit 600, and outputs a resulting feedback signal to the PWM IC 410. The PWM IC 410 then controls a duty ratio of the PWM signal according to the feedback signal input thereto, thus maintaining the output voltages Vcc1 and Vcc2 at a regular voltage level.
Herein, the frequency of the PWM signal of the PWM IC 410 should be equal to the frequency of the horizontal sync signal provided by a computer main body, DVD player, etc. If the PWM signal frequency of the PWM IC 410 and the frequency of the horizontal sync signal are not equal to each other, the display apparatus displays noise. To equalize the PWM signal frequency and the horizontal sync signal frequency, the AFC signal that is supplied by a horizontal deflection circuit equipped to the display apparatus is applied to a sync signal input terminal of the PWM IC 410 through the sync signal supplier 800.
Provided that about 40V of the AFC signal output from the horizontal deflection circuit is converted into about 4˜5V of sync signal through a sync signal transformer (hereinbelow, “a sync trans”) and the converted sync signal is supplied to the PWM IC 410 through capacitors C1 and C2, a resistor R1, and diodes D2 and ZD5, the PWM IC 410 generates a PWM signal synchronized to the sync signal input thereto, thereby turning on/off the transistor.
In a power supply system of a conventional display apparatus the horizontal deflection circuit does not work when the display apparatus is switched from a normal operation mode to a power-off mode. When in the power-off mode, the sync signal cannot be supplied to the PWM IC 410 because no AFC signal exists. The microcomputer 700 provides a power-off mode signal to the feedback circuit which turns off the power by controlling an operation of the feedback circuit.
The power supply system of the conventional display apparatus has a disadvantage of added cost from complex circuitry to supply the AFC signal from the horizontal deflection circuit and the voltage (14V) of the secondary terminal of the switching transformer 500 to the sync trans. Moreover, it is difficult to lower the power consumption of the complex circuitry below 1 or 2 watts.
Additionally, the complex circuitry of a conventional display apparatus causes a delay in supplying the sync signal to the PWM IC 410 when the display apparatus is switched from a power saving mode, such as the power-off mode to the normal operation mode. Thus, the power falls below a threshold level and the microcomputer 700 needs to be reset due to the mistiming of the supply power output through the output circuit 600.